Gravity is the force of attraction between objects. Gravity is that force of attraction that will cause hydrogen gas in a nebula to clump together overtime. These hydrogen atoms will form a spinning cloud of gas within the nebula.
As collisions between the atoms become more frequent, the temperature within the cloud will rise.
As the cloud gets more massive the greater the force of gravity will be and so it will attract even more gas to the spinning cloud. When the temperature reaches fifteen million degrees Celsius, nuclear fission begins and a star is born.